Many processes have been disclosed in which transition metal catalysts are used to carbonylate olefins. In particular, palladium-catalyzed carboalkoxylation reactions have been studied for many years as a means of converting butadiene to adipic acid precursors. Catalyst systems for such processes which comprise palladium and monodentate or multidentate Group V ligands are disclosed in: German Offenlegungsschrift No. 2410246; EP 55,875; EP 273,489; EP 284,170; and U.S. Pat. No. 3,887,595. Catalyst systems which comprise palladium, a Group V ligand, and an acid promoter are disclosed in: U.S. Pat. No. 3,437,676; U.S. Pat. No. 3,501,518; U.S. Pat. No. 4,414,409; U.S. Pat. No. 4,786,443; G.B. 1,110,405; EP 43,382; EP 106,379; EP 198,521; EP 227,160; EP 235,864; EP 271,145; and EP 279,477.
U.S. Pat. No. 4,172,087 discloses a palladium catalyst system consisting of: (a) one or more palladium halide salts in combination with one or more monodentate tertiary phosphorus containing donor ligands, or one or more palladium halide free salts in combination with one or more multidentate, tertiary phosphorus containing donor ligands; (b) at least one molar equivalent of a hydroxyl group containing co-reactant; and (c) a nitrogen-containing base. The preferred starting material is 1,3-butadiene. Pyridine, alkylated pyridines, quinoline, lutidine, picoline, isoquinoline, alkylated quinolines and isoquinolines, acridine, N,N-dimethylaniline, N,N-diethylaniline, N,N-diethyltoluidine, N,N-dibutyltoluidine, N,N-dimethylformamide and N-methyl-2-pyrrolidone can be used as the nitrogen-containing base.
U.S. Pat. No. 3,437,676 discloses a carbonylation process in which an olefinically unsaturated compound is reacted with carbon monoxide and an alcohol to produce a carboxylic acid ester, the reaction being carried out under the influence of a complex palladium salt as a catalyst in an amount of 0.01 to 1% by weight with reference to the olefinically unsaturated compound. Triphenylphosphinepyridine palladium dichloride is a suitable catalyst (Col. 4, lines 3-4). The reaction may be carried out in the presence of an organic or inorganic acid, including sulfuric acid, phosphoric acid, boric acid, acetic acid, propionic acid, other carboxylic acids, or halogen hydracids in amounts up to 10% by weight with reference to the whole of the initial materials (Col. 4, line 63-Col. 5, line 1).
The reactions disclosed in the prior art for the carboalkoxylation of butadiene often proceed at moderate temperatures and pressures, and thus provide attractive routes to dimethyl adipate, a precursor to adipic acid. However, the selectivity to the desired linear products (dialkyl adipates) achieved by these reactions is usually less than 75%, and hence there is a need to develop more selective processes for the carboalkoxylation of butadiene.
It has now been found that a catalyst comprising palladium, chloride, a strong acid, an aryl phosphine, and a N-heterocyclic base can be used in the carboalkoxylation of butadiene to give higher butadiene conversion and greater selectivity to the desired pentenoate esters.